Recently, a coherent optical communication system has been expected as one of long distance and large capacitance communication systems, and has been developed rapidly. In such a coherent optical communication system, a CP (continuous phase) FSK heterodyne method has been used generally.
A conventional optical circuit used for a polarization diversity receiver includes a polarization beam splitter (PBS) for splitting an input signal light to provide two polarization lights, a first optical coupler for branching a local signal light to two signal lights, and second and third optical couplers for coupling one of output signals of the PBS and one of output signals of the first optical coupler, respectively.
Output signals of the second and third optical couplers are supplied to two optical double-balanced receivers, and the optical signals are converted to electric signals thereby, respectively.
According to the conventional optical circuit, however, there is a disadvantage in that two heterodyne receiver including the two optical double-balanced receivers are required in the optical receiver. Therefore, the optical receiver in which the conventional optical circuit is used as a front-end is difficult to be simplified in structure.